The invention relates to an operating method for a cryopressure tank in which cryogenic hydrogen for supplying a consumer, in particular a fuel cell of a motor vehicle, can be stored under supercritical pressure at 13 bar or more. For compensating the pressure drop resulting from the removal of hydrogen from the cryopressure tank, either a heat transfer medium is supplied via a control valve to a heat exchanger provided in the cryopressure tank or no supply of heat transfer medium takes place into said heat exchanger.
With regard to the prior art, reference is made to EP 2 217 845 B1 in which possible embodiments of an operating method for a cryopressure tank and also the so-called cryogenic storage in particular of hydrogen is described. As further relevant prior art, in addition to U.S. Pat. No. 6,708,502 B1, in particular German patent applications 10 2007 011 530.1 and 10 2007 011 742.8 are cited. In these documents it is described that in a so-called cryopressure tank cryogenic hydrogen can be stored at supercritical pressure values, namely at 150 bar and more, or, respectively, that in the course of refueling, such hydrogen can be filled at supercritical pressure into a suitable cryopressure tank, which offers significant advantages.
In order to be able to remove the hydrogen from the cryopressure tank with a certain minimum pressure level for supplying the consumer, this minimum pressure level is set or maintained by a defined heat input into the cryopressure tank, which, with the method according to EP 2 217 845 B1 is carried out in that a portion of the hydrogen removed from the cryopressure tank, after heating the same, is fed through an (internal) heat exchanger provided in the cryopressure tank, wherein it is also described in this document how such a minimum pressure level can be set with simple control measures and also with regard to the object of being able to empty the cryopressure tank as completely as possible.
For an operating method for a cryopressure tank, in which, deviating from the above-mentioned EP 2 217 845 B1, in addition to the removed hydrogen, another (any other) heat transfer medium can also be used for supplying heat into the cryopressure tank. The present object is to provide an improvement to the effect that the heat input into the cryopressure tank carried out for compensating the pressure drop resulting from the removal of hydrogen is kept as low as possible.
The solution to this object is characterized in that depending on the fill level of the cryopressure tank, the control valve is actuated based on different criteria, namely such that in the case of a higher filling level and as long as there is a risk of liquefaction of the residual hydrogen in the cryopressure tank during removal of cryogenic hydrogen from the cryopressure tank, the temperature of the hydrogen in the tank does not drop below the critical temperature (hereafter also designated as “temperature criterion”), while in the case of a low fill level, when there is no longer the risk of liquefaction during the removal of cryogenic hydrogen and as long as this is possible with the residual amount of hydrogen contained in the cryopressure tank, the pressure in the cryopressure tank is adjusted such that it does not drop below a minimum pressure value which the hydrogen removed from the cryopressure tank must have to be usable in the consumer without limiting the function of the latter (hereafter also designated as “minimum pressure criterion”).
Through heat input—as already explained—pressure is built up in the cryopressure tank, which pressure ensures supply to the consumer. A heat input as low as possible into the cryopressure tank is particularly advantageous if only a so-called “single-flow” refueling or filling of the tank is intended, i.e., if it is not possible to flush the cryopressure tank during the refueling/filling process first with cold hydrogen which, after sufficient cooling of the cryopressure tank, is discharged again from the cryopressure tank prior to the actual filling.
It is preferred, in any case when heat supply into the cryopressure tank by means of the heat transfer medium is carried out only for meeting the above-mentioned temperature criterion, i.e., for maintaining a hydrogen temperature value of greater than 33.18 Kelvin, to reduce the heat supply as far as possible with regard to down times and tolerances. It is therefore preferred to keep the temperature of the hydrogen in the cryopressure tank as close as possible to the critical temperature (of 33.18 Kelvin) and thus, for example, in a range between 34 Kelvin and 40 Kelvin, as soon as or as long as during removal of cryogenic hydrogen from the tank there is a risk of liquefaction of the residual hydrogen remaining in the tank. Thus, the heat input into the cryopressure tank is kept as low as practically possible, wherein it is to be explicitly mentioned that in the case of a completely filled cryopressure tank it is of course possible that such a high pressure (for example of the order of 300 bar) and such a relatively high temperature (for example of the order of 50 Kelvin) prevails that by removing cryogenic hydrogen there is in first instance no risk of liquefaction, and a heat input into the cryopressure tank controlled according to the invention is therefore not required. Rather, in the case of an at least approximately completely filled tank, in first instance, it is only monitored that the mentioned temperature criterion is met.
In the present case, as it were, sequential temperature control and pressure control or, more precisely, feedback control is proposed, wherein above a certain fill level of the cryopressure tank, which, in the case of complete filling can be filled up to a pressure of, for example, 300 bar, a control strategy that can be provided in a simple manner including an easily measurable control variable, namely the temperature of the cryogenic hydrogen in the cryopressure tank, is used. It is ensured here that the temperature, in particular in connection with removal of hydrogen from the cryopressure tank, does not drop below the temperature of the “critical point” (=33.18 Kelvin), so that the desired supercritical state of the stored hydrogen is reliably maintained despite the removal of hydrogen. In fact, it is undesirable that hydrogen stored in the supercritical state in the cryopressure tank transitions into a state in the so-called two-phase region in which both liquid and gaseous hydrogen can be present. For the sake of completeness it should be noted that without removing hydrogen from the cryopressure tank, the risk that the hydrogen in the cryopressure tank cools down to an extent that the hydrogen partly or completely transitions into the undesirable liquid aggregate state does not exist due to the unavoidable minimal continuous heat input into the cryopressure tank through the tank wall thereof.
When the fill level of the cryopressure tank has dropped to such an extent that further removal of hydrogen from the tank no longer poses the risk of liquefaction of the hydrogen in the tank, the temperature criterion no longer needs to be taken into account. However, upon further removal of hydrogen, the pressure in the tank can fall relatively fast to such a low value (for example, of the order of 2 bar) that sufficient supply to the consumer would then no longer be ensured. For this reason, according to the invention, a switch is made to a pressure control to the effect that the pressure in the tank does not drop below a minimum pressure which the hydrogen that is removed from the tank must have so as to be usable in the consumer without limiting the function thereof. Hereby, the operating state of the cryopressure tank in which, according to the invention, a switch is made from the (described) temperature control to the (described) pressure control is basically described with sufficient accuracy by the criterion whether, in particular when removing hydrogen from the cryopressure tank, there is the risk of liquefaction of the hydrogen in the tank. As is well known, this risk only exists if the temperature of the hydrogen stored in the cryopressure tank is below the critical temperature (of 33.18 Kelvin) and, at the same time, the pressure is below the critical pressure (of 13 bar).
Within the context of meeting the above-mentioned temperature criterion, different control strategies, again in dependence on the fill level of the tank, can be of advantage. It has already been mentioned that in a completely filled cryopressure tank, a pressure of, for example, 300 bar and a temperature of, for example, 50 Kelvin can prevail therein.
If, starting from this state and upon removal of hydrogen from the tank, in the first instance a pressure drop and cooling-down of hydrogen remaining in the tank takes place, then, in this case, only the mentioned temperature criterion is relevant. Preferably, the heat input into the tank is kept low such that the temperature of 40 K is not exceeded. According to the known pressure-density-diagram of hydrogen (in particular in the proximity of the critical point), which is illustrated in part in the attached FIG. 5 and in DE 10 2007 011 530 A1 as FIG. 1, this results in a pressure in the tank of at least 13 bar. In the case of a still approximately completely filled cryopressure tank, the pressure in the tank is still significantly higher; however, with further reduction of the fill level of the tank, the pressure in the tank will approach this critical pressure of 13 bar (12.81 bar, to be exact). In the case of a fill level of this order of magnitude of slightly more than 13 bar and a temperature in the mentioned order of magnitude of 33 K to 40 K, it is therefore reasonable to input only so much heat or, better, only so little heat into the tank that the pressure in the cryopressure tank, while meeting the above-mentioned criterion which, in first instance, is still used as priority control variable, is or remains in the value range between 13 bar and 15 bar.
The heat input into the tank that, as it were, is added up over the consumption of a complete tank filling can be further minimized if at and onwards an operating state or fill state of the tank at which, while meeting the above-mentioned temperature criterion, the pressure in the tank can be lowered to values below the critical pressure, this pressure of the stored hydrogen is also lowered. With this in mind, it is then possible, if removing of hydrogen from the tank while supplying heat into the tank in view of maintaining a pressure of 13 bar to 15 bar and, at the same time, meeting the mentioned temperature criterion, results in a significant temperature increase of the hydrogen in the tank, to adjust the pressure in the tank to a lower value in the order of magnitude of 3 bar to 7 bar, which is still sufficient for supplying a fuel cell system or an internal combustion engine that is not charged or only to a minor extent.
With further reduction of the fill level, i.e., after further removal of hydrogen from the tank with the tank still being filled, for example, to one third, it could happen that by applying or meeting only the above-mentioned temperature criterion, finally a pressure occurs that not only lies below the pressure in the critical point, but can also drop rapidly to values below 3 bar. The stored hydrogen would still be in the gaseous state, as desired, but reliable supply of hydrogen from the cryopressure tank to the consumer would no longer be ensured by such a low pressure. Therefore, when removing hydrogen from the tank, heat is introduced into the tank only to such an extent that a pressure value in the order of magnitude of 3 bar to 7 bar is maintained, which is required for sufficient supply to the consumer, in particular to a fuel cell system. Through this, the state of pure pressure control (basically already described above) is achieved, in which the mentioned temperature criterion needs no longer to be taken into account since with decreasing filling quantity and thus dropping density, the temperature in the tank cannot fall below the critical temperature of 33.18 K any more. Rather, the mentioned minimum pressure criterion alone serves now for controlling the heat input into the cryopressure tank, wherein this pressure control is designed in such a manner that the control variable, namely the pressure of the hydrogen contained in the cryopressure tank, does not drop below a minimum value which is needed for reliably supplying a consumer without limiting the function thereof. As already described, this minimum value can lie in the order of magnitude of 3 bar to 7 bar and, preferably, in the order of magnitude of 5 bar (absolute), since therewith, depending on the losses of a line system with integrated regulators between the cryopressure tank and a fuel cell system (as an exemplary consumer), this fuel cell system can be reliably operated.
In other advantageous embodiments, analogous to the above-mentioned EP 2 217 845 B1, fuel removed from the tank and heated in an heat exchanger can be used as a heat transfer medium and can be supplied to the heat exchanger via a branch line branching off a supply line that leads to the consumer, and after flowing through the heat exchanger, it can be fed into the supply line downstream of the branching-off point of the branch line. Here, downstream of the above-mentioned branching-off point of the branch line, a pressure control unit can be provided in the supply line. The pressure control unit ensures that continuous supply of hydrogen at the required pressure to the consumer is ensured despite the changes in pressure in the supply line caused upstream of the pressure control unit by the switching of the control valve and defines the magnitude of the heat input into the tank (by actuating an electronic control unit). Furthermore, the control valve can be closed or opened over a time period that significantly exceeds the cycle times of a conventional cycle valve so as to keep the control process simple and also to keep the mechanical load on the control valve low.
According to an advantageous refinement of the operating method according to the invention, the waste heat of a fuel cell system as a consumer supplied from the tank can be fed at least in part to the mentioned heat transfer medium, which is known for a cryo-adsorptive storage device for hydrogen from DE 10 2011 017 206 A1. For minimizing the heat input into the cryopressure tank, it is proposed in the present case to carry out a partial heat dissipation from the fuel cell system into the tank only if this is necessary for satisfying a higher cooling capacity demand of the fuel cell system, which demand cannot be completely covered by the actual cooling system thereof which ultimately uses the surrounding air as a temperature sink. Depending on the demand, heat dissipation from the fuel cell system via the heat transfer medium with heat supply into the cryopressure tank is carried out only until the hydrogen in the tank has a predefined maximum pressure as a result of this heat supply. This predefined maximum pressure is designed in view of the maximum pressure allowable for the operational safety of the tank.
With the measures proposed here and, in particular, with the temperature and pressure control disclosed, it is principally possible to achieve a heat input into the cryopressure tank that is significantly reduced with respect to the prior art, as is apparent from the further explanation of two exemplary embodiments.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.